1. Field of the Invention
The present invention relates to a method for making an active material and electrode to be used in rechargeable electrochemical elements such as lithium ion secondary batteries and electrical double layer capacitors.
2. Related Background Art
Rechargeable electrochemical elements such as lithium ion secondary batteries and electrical double layer capacitors (EDLC) are widely used or under research and development for cellular phones, laptop computers, PDAs, automobiles and the like. The major positive electrode active materials in lithium ion secondary batteries are LiCoO2, LiNixCo1-xO2, LiMn2O4, LiCoxNiyMn1-x-yO2 and LiCoxNiyAl1-x-yO2. Primarily employed or researched as negative electrode active materials are carbonaceous materials such as artificial graphite, natural graphite, mesocarbon microbeads (MCMB), coke, fibrous carbon, surface-modified carbon and the like, as well as tin compounds and silicon compounds. The maximum charging voltage of batteries combining these positive electrode active materials and negative electrode active materials is 4.1-4.2 V, and the energy density is at most 400-500 Wh/L.
Along with the increase in energy consumption of appliances in recent years, there has been a demand for even higher energy density for batteries. However, it is becoming difficult to obtain greater energy density through battery design optimization (reducing the thickness of the container housing the constituent elements of the battery, or reducing the thickness of the positive/negative electrode collectors and separator).
One method of realizing higher energy density utilizes the capacity of the high potential sections of the positive electrode active material, rather than the potential in the regions conventionally used for charge/discharge. In other words, this method attempts to increase the energy density by raising the charging voltage of the battery. For example, LiCoxNiyMn1-x-yO2 can raise the discharge capacity by increasing the charging voltage (−4.6 V vs Li/Li+) above the conventional charging voltage (4.2 V-4.3 V vs Li/Li+), thus allowing the energy density to be increased.
However, increasing the charging voltage can lead to new problems such as reduced cycle life and storage characteristics of the battery (due to decomposition of the electrolyte solution/electrolytes/positive electrode active material), and lower thermostability of the battery (due to a lower exothermic peak temperature or greater heat release of the positive electrode active material). Methods of covering positive electrode active material surfaces with oxides have been disclosed as a way of avoiding these problems (Japanese Unexamined Patent Publication HEI No. 07-288127, Japanese Unexamined Patent Publication HEI No. 04-319260 (Japanese Patent No. 2855877), Japanese Unexamined Patent Publication No. 2005-85635, Japanese Unexamined Patent Publication No. 2000-200605, Japanese Unexamined Patent Publication No. 2006-107763, Japanese Unexamined Patent Publication No. 2005-276454, Japanese Unexamined Patent Publication No. 2006-156032, Japanese Unexamined Patent Publication No. 2007-018743, Japanese Unexamined Patent Publication No. 2003-109599, Domestic Re-publication of PCT International Application No. 03-069702, Japanese Unexamined Patent Publication No. 2003-331846, Japanese Unexamined Patent Publication No. 2005-085471, Electrochemical and Solid-State Letters. 6 (11) A221-A224 (2003), Electrochimica Acta 49 (2004) 1079-1090, Electrochemical and Solid-State Letters. 6(1) A16-A18 (2003)).